Titanium thin sheet

ABSTRACT

A titanium thin sheet of 0.2 mm or less in thickness, containing: Fe of 0.1 mass % or less and O (oxygen) of 0.1 mass % or less in a bulk, wherein a sheet thickness (mm)/a grain size (mm) ≧3, and the grain size ≧2.5 μm are satisfied, and a hardened layer is included at a surface, and a region of the hardened layer is a depth of 200 nm or more and 2 μm or less from the surface. The titanium thin sheet is supplied with excellent workability and high surface hardness, and is able to be suitably used for various purposes such as, for example, acoustic components (a speaker vibration plate and so on).

TECHNICAL FIELD

The present invention relates to a titanium thin sheet, in more detail,to a high-strength titanium thin sheet having excellent workability andhigh surface hardness, and excellent in workability capable of suitablybeing used for a speaker vibration plate and so on. This application isbased upon and claims the benefit of priority of the prior JapanesePatent Application No. 2012-179861, filed on Aug. 14, 2012, the entirecontents of which are incorporated herein by reference.

BACKGROUND ART

A titanium material has high specific strength and excellent corrosionresistance, and is used for various purposes as industrial raw materialsfor a chemical plant, for architecture, and for many others, or asmaterials of consumer products such as camera bodies, clocks, sportsequipments. A thin sheet such as a foil of 0.2 mm or less in thicknessis used for purposes making use of characteristics thereof such asacoustic components (a speaker vibration plate and so on), ananticorrosive film, sheet.

In general, there is a tendency in which high-strength is required formetal materials, and in addition, workability is also required. Thetitanium material is no exception, and it is often the case in which thehigh-strength is also required in addition to the excellent workability.However, in general, the workability is lowered when it is highlystrengthened, and therefore, in the titanium material, an attempt tooptimize a balance between strength and workability has been done bycontrolling an oxygen amount, an iron amount, a crystal grain size, andso on.

For example, in Patent Document 1, a titanium sheet in which strength isimproved while suppressing lowering of ductility of the titanium sheetby increasing an Fe content (Fe: 0.1 mass % to 0.6 mass %) while settingan O (oxygen) content in the titanium material at a predetermined value,and formability is improved by setting an average grain size to be 10 μmor less is disclosed.

In Patent Document 2, a Ti sheet material having fine formingworkability whose nitrogen amount and hydrogen amount are limited inaddition to an iron amount and an oxygen amount such that the Fe contentis 300 ppm or more and a [Fe+O+N+H] amount is 1500 ppm or less isdisclosed.

Besides, in Patent Document 3, a manufacturing method of a pure titaniumsheet in which an iron amount, an oxygen amount, further nickel andchromium amounts are specified into a predetermined range and an averagegrain size is set to be 20 μm to 80 μm to keep fine formability evenwhen a cheap raw material whose purity is low is used is disclosed.

However, all of the arts described in these Patent Documents are artswhose target is a general titanium material whose of 0.3 mm to 1 mm inthickness.

On the other hand, a thin sheet and a foil of 0.2 mm or less inthickness used for the speaker vibration plate and so on is thinner thana material for general purposes, and it is inferior in workability.Accordingly, there is a problem in which working failure occurs even ifthe arts described in the above-stated Patent Documents 1 to 3 areapplied.

As for the workability of the titanium thin sheet of 0.2 mm or less inthickness, a manufacturing method of a titanium foil excellent informability is disclosed in Patent Document 4. According to this art, atitanium foil of 25 μm in thickness is rolled under a predeterminedrolling condition, and a crystal grain size is controlled to be ASTM No.12 to 14, and thereby, the fine Erichsen value is secured.

However, in the titanium foil of 0.2 mm in thickness or less, fine shaperetentivity after the forming work is required. In general, strength ofa material is improved, and thereby, the fine shape retentivity issecured, but at the same time, there is a problem in which fineworkability cannot be obtained. Besides, a part where a large work isperformed improves in strength by work hardening and the fine shaperetentivity can be obtained, but a part where working ratio is low isinferior in the shape retentinity.

For example, in Patent Document 5, an art in which a layer containing acarbide and/or nitride of titanium is formed as an inner surface layerby a bright annealing or a vacuum annealing, and thereafter, anelectrolytic acid pickling is performed is disclosed. This art is one inwhich a contact between a soft titanium base material and a die issuppressed, to thereby prevent an adhesion of the titanium base materialto the die, and at the same time, to form an oxide layer excellent inlubricity at a press time at a surface of titanium. According to thisart, it is possible to avoid that the carbide and/or nitride of titaniumis in contact with the die, and wear of the die is prevented.

However, it is a rare case in which a severe work as disclosed in thePatent Document 5 is performed for the titanium foil of 0.2 mm or lessin thickness. For example, in the work of the speaker vibration plateand so on, it is often the case in which an internal pressure is appliedto form into a dome state, and a possibility of the contact with the dieduring the work is small compared to a general forming by a thin-sheetpress, and a surface lubricity of the material in itself is not such aproblem. Accordingly, an workability improvement effect owing to alubricating effect of the oxide is not exhibited even if the artdescribed in the Patent Document 5 is applied. Further, in the art, theelectrolytic acid pickling is performed, and therefore, lowering ofyield when the art is applied for the titanium foil material of 0.2 mmor less in thickness cannot be overlooked. In addition, there is a casewhen shipment as a product becomes impossible caused by unevenness ofthe sheet thickness.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Patent Publication No. 4605514

Patent Document 2: Japanese Laid-open Patent Publication No. S63-103043

Patent Document 3: Japanese Patent Publication No. 3228134

Patent Document 4: Japanese Patent Publication No. 2616181

Patent Document 5: Japanese Laid-open Patent Publication No. 2009-97060

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention is made in consideration of circumstances asstated above, and an object thereof is to provide a titanium thin sheetof 0.2 mm or less in thickness, and excellent in shape retentivity andworkability.

Means for Solving the Problems

To solve the above-stated problems, the present inventors focusattention on surface hardness of the titanium foil, and think that it ispossible to enable both the shape retentivity and the workability if thesurface is hard and an inner side is soft compared to the surface, andstudy about a method to improve the workability and the surface hardnessof the titanium thin sheet.

As an effective method to improve the workability of the titanium thinsheet, at first, it is conceivable to reduce elements such as iron andoxygen. These elements are elements inevitably introduced atmanufacturing time, but as described in the Patent Documents 1 to 3, itis necessary to limit to a predetermined amount or less.

Next, it is conceivable to make crystal grains coarse. It is possible tomake a twinning deformation which is important for the workability ofthe titanium material easily occur by making the grain coarse, and theworkability is improved. A crystal grain size is controlled at a finishannealing process at the last, and therefore, it is easily controlled bychanging annealing conditions.

A tensile test is performed by using a titanium thin sheet with a sheetthickness of 0.2 mm or less to investigate elongation. As a result, theelongation is lowered by refining the crystal grain as same as a generalknowledge also in the sheet thickness of 0.2 mm or less. However, itturns out that there is a case when the elongation is lowered if thecrystal grain becomes too coarse in the titanium thin sheet of 0.2 mm orless in thickness. Besides, whether or not this phenomenon occurs isdetermined by a ratio between the sheet thickness and the grain size,and it turns out that this phenomenon occurs when the sheetthickness/the grain size <3. Note that in case of the thin sheet ofapproximately 0.3 mm to 1 mm in thickness, the phenomenon in which theelongation is lowered by the coarseness of the crystal grain does notoccur because the grain size is approximately within a range of 10 μm to60 μm.

From this investigation result, the crystal grain is made coarse withina range in which the sheet thickness/the grain size ≧3 in accordancewith the product sheet thickness, and thereby, it becomes possible toexploit the workability of the titanium thin sheet of 0.2 mm or less inthickness to the maximum.

In a process further advancing the investigation, there is a case whencracks frequently occur at a press working time, and a cause thereof isinvestigated, then it turns out that a carbon amount and a nitrogenamount in a vicinity of a material surface are high at a part where thecracks occur. Normally, when the thin sheet of 0.2 mm or less inthickness is manufactured, a bright annealing (BA) to give formabilityand workability by softening is performed after a cold-rolling. However,when removal of rolling oil at a cleaning line before the annealing isinsufficient, a lot of rolling oil remains at the material surface, andan entering amount of carbon in the vicinity of the material surfacebecomes large. Nitrogen is nitrogen gas remained at a gas exchange timeof an annealing furnace, and when the exchange is insufficient, a lot ofnitrogen remains, and an entering amount of nitrogen becomes large.

The entered carbon, nitrogen form TiC, TiN, incur solid-solutionstrengthening, and therefore, the surface hardness becomes high, and theshape retentivity becomes good also in an ultrathin shape titanium thinsheet of 0.2 mm or less in thickness. However, when they enter too deep,the elongation of the material is remarkably lowered. It is necessary toset entering depths of carbon, nitrogen, oxygen to be within a range of200 nm to 2 μm from a surface to enable the above-stated bothcharacteristics (namely, the improvement in the surface hardness and thesuppression of the elongation lowering). Namely, it is necessary that aregion of a hardened layer formed by the entering of carbon, nitrogen,oxygen is to be within the range of 200 nm to 2 μm from the surface.

The present invention is made based on the studied information, and acontent thereof is a high-strength titanium thin sheet excellent inworkability described below.

Namely, it is a titanium thin sheet of 0.2 mm or less in thickness,which contains Fe of 0.1 mass % or less and O (oxygen) of 0.1 mass % orless in a bulk, satisfies a sheet thickness (mm)/a grain size (mm) ≧3,and a grain size ≧2.5 μm, includes a hardened layer at a surface, and aregion of the hardened layer is at a depth of 200 nm or more and 2 μm orless from the surface.

After a cold-rolling, it is desirable if a finish annealing (brightannealing) is performed for the titanium thin sheet of the presentinvention at 500° C. or more and 850° C. or less by a BAF (batch heattreatment) or a continuous annealing because stable workability isthereby secured.

The “titanium thin sheet” described here means industrial pure titaniumdefined by JISH4600, and a thin sheet or a foil of 0.2 mm or less inthickness.

The “grain size” means an average grain size found by a quadraturedefined by JISH0501. There is a case when it is described as an “averagegrain size” with an emphasis on the above.

Besides, the “hardened layer” is a concentrated layer of oxygen,nitrogen, carbon formed at an annealing time by carbon, nitrogen andoxygen comes from rolling oil remaining at a surface, and nitrogen andoxygen gas contained in a gas atmosphere of an annealing furnace.

Effect of the Invention

A titanium thin sheet of the present invention is a titanium thin sheetof 0.2 mm or less in thickness, where excellent workability and highsurface hardness are given, and is a titanium thin sheet (foil) capableof suitably being used for various purposes such as, for example,acoustic components (speaker vibration plate, and so on).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view exemplifying a relationship between a crystal grainsize and elongation in a tensile test of a titanium thin sheet.

FIG. 2 is a view exemplifying a relationship between a stress and astrain in the tensile test of a titanium thin sheet (foil) with athickness of 25 μm.

FIG. 3 is a view exemplifying a relationship between a sheet thickness/agrain size and elongation in the tensile test of the titanium thinsheet.

FIG. 4 is a view illustrating a relationship between a hardened layerthickness and a surface hardness at the titanium thin sheet.

FIG. 5 is a view exemplifying a relationship between a hardened layerthickness and elongation at the titanium thin sheet of 100 μm inthickness, and a sheet thickness/a grain size ≧3.

MODE FOR CARRYING OUT THE INVENTION

A titanium thin sheet of the present invention is a titanium thin sheetof 0.2 mm or less in thickness, which contains Fe of 0.1 mass % or lessand O (oxygen) of 0.1 mass % or less in a bulk, satisfies a sheetthickness (mm)/a grain size (mm) ≧3, and a grain size ≧2.5 μm, includesa hardened layer at a surface, and a region of the hardened layer is ata depth of 200 nm or more and 2 μm or less from the surface.

In the present invention, a reason why the titanium thin sheet of 0.2 mmor less in thickness is intended for is to provide a high-strengthtitanium thin sheet excellent in workability capable of suitably beingused also for, for example, the speaker vibration plate and so on.

In the titanium thin sheet of the present invention, a reason why thebulk Fe is defined to be 0.1 mass % or less is as described below.Namely, Fe is an element stabilizing β-phase, and when the β-phaseexists, a growth of a crystal grain is disturbed by the β-phase duringan annealing. When a content exceeds 0.1 mass %, a function thereofbecomes remarkable, and therefore, the content of Fe is set to be 0.1mass % or less. A lower limit is not particularly limited, but mixtureof Fe is inevitable when it is industrially manufactured, and 0.01 mass% or more is contained, and therefore, a desirable lower limit is set tobe 0.01 mass %.

Besides, a reason why the bulk O (oxygen) is defined to be 0.1 mass % orless is to suppress lowering of workability. O is added, and thereby,the titanium thin sheet is highly strengthened, but the workability islowered, and when a content exceeds 0.1 mass %, a tendency thereofbecomes remarkable, and therefore, the content of O is set to be 0.1mass % or less. A lower limit is not particularly limited, but mixtureof O is inevitable when it is industrially manufactured as same as Fe,and therefore, a desirable lower limit is set to be 0.01 mass %.

Note that the bulk means an inside of the titanium thin sheet except thehardened layer formed at a surface of the titanium thin sheet. In thepresent invention, an Fe concentration is 0.1 mass or less, and an Oconcentration is 0.1 mass % or less in the bulk.

In the titanium thin sheet of the present invention, a reason why thegrain size ≧2.5 μm is to be satisfied is that when the grain size isless than 2.5 μm, the elongation is largely lowered, and it is inferiorin the workability as illustrated in FIG. 1.

FIG. 1 is a view exemplifying a relationship between the crystal grainsize and the elongation in a tensile test of the titanium thin sheet. Asillustrated in the drawing, when the crystal grain size is less than 2.5μm, it becomes too high-strengthened even if a non-recrystallized graindoes not exist, and therefore, the elongation is largely lowered.

In the titanium thin sheet of the present invention, a reason why thesheet thickness (mm)/the grain size (mm) ≧3 (hereinafter, “the sheetthickness (mm)/the grain size (mm)” is just referred to as “the sheetthickness/the grain size”) is to be satisfied is as described below.

FIG. 2 is a view exemplifying a relationship between a stress and astrain in the tensile test of a titanium thin sheet (foil) of 25 μm inthickness. In the drawing, “the grain size: 5.3 μm” and “the grain size:12.3 μm” are measurement results as for test pieces whose average grainsizes are respectively 5.3 μm and 12.3 μm.

As illustrated in FIG. 2, in any of the test pieces, after passingthrough a uniform elongation state, it starts local deformation, andreaches a fracture. A local deformation amount is small, and a uniformdeformation amount, namely, the uniform elongation is an index of theworkability, and when the uniform elongation is lowered, it means thelowering of the workability.

In a deformation of a polycrystalline material, when one grain deforms,relaxation of deformation occurs by crystal grains at a peripherythereof. However, when the number of crystal grains are small relativeto a sheet thickness direction, a contribution to the deformation of onecrystal grain becomes large, the deformation progresses at a specificcrystal grain, and therefore, the local deformation early starts. FIG. 2illustrates this state.

Accordingly, by the number of crystal grains existing in the sheetthickness direction, namely, by the ratio of the sheet thickness/thegrain size, an upper limit of a range of an average crystal grain sizecapable of improving the workability by coarsening is determined.

FIG. 3 is a view exemplifying a relationship between the sheetthickness/the grain size and the elongation in the tensile test of thetitanium thin sheet. As illustrated in FIG. 3, the elongation isremarkably lowered at around the sheet thickness/the grain size=3 in anyof the titanium thin sheets of 25 μm to 150 μm in thickness, and it canbe seen that it is necessary to satisfy the sheet thickness (mm)/thegrain size (mm) ≧3.

Further, in the titanium thin sheet of the present invention, it isnecessary to have the hardened layer at the region at the depth of 200nm or more and 2 μm or less from the surface. In other words, it isnecessary to have the hardened layer of 200 nm to 2 μm in thickness inthe vicinity of the surface.

The hardened layer is the concentrated layer of oxygen, nitrogen andcarbon formed at the annealing time by carbon, nitrogen and oxygen comesfrom the rolling oil remaining at the surface, nitrogen and oxygen gascontained in the gas atmosphere of the annealing furnace, and is aregion containing oxygen of 0.5 mass % or more, a region containingnitrogen of 0.5 mass % or more, a region containing carbon of 0.5 mass %or more, or a region containing oxygen, nitrogen, and carbon of 0.5 mass% or more as a total. Note that the thickness of the hardened layer isable to be measured by a GDS (Glow discharge optical emissionspectrometer).

FIG. 4 is a view illustrating a relationship between the hardened layerthickness and the surface hardness in the titanium thin sheet. Asillustrated in FIG. 4, the thicker the thickness of the hardened layeris, the higher the surface hardness becomes. When the thickness of thehardened layer is thinner than 200 nm, it is the same degree as amaterial hardness (illustrated in FIG. 4) measured at a cross section ofthe material, and an increase of the hardness is not recognized.Besides, when the increase of the surface hardness is insufficient, itis inferior in the shape retentivity. Accordingly, the thickness of thehardened layer is set to be 200 nm or more.

FIG. 5 is a view exemplifying a relationship between the hardened layerthickness and the elongation in the titanium thin sheet of 100 μm inthickness, and the sheet thickness/the grain size ≧3. As illustrated inFIG. 5, even when the sheet thickness/the grain size ≧3, the elongationis lowered if the hardened layer thickness is too thick to lead to thelowering of the workability, and therefore, the hardened layer thicknessis set to be 2000 nm (2 μm) or less.

The titanium thin sheet of the present invention is desirable if thefinish annealing is performed at 500° C. or more and 850° C. or less bythe BAF or the continuous annealing after the cold-rolling, becausestable workability is secured.

When an annealing temperature is low, the non-recrystallized grainremains, and the workability is lowered. A recrystallization temperatureof the titanium thin sheet of the present invention is 500° C., andtherefore, the finish annealing is performed at 500° C. or more.Besides, the finish annealing is performed at 850° C. or less to obtainan equiaxed structure in which a balance between excellent strength andductility (elongation) is easy to obtain. An operation in accordancewith an object of an annealing process is performed also in a normaloperation, but the workability is stably secured by performing thefinish annealing under the above-stated desirable temperature condition.

The thickness of the hardened layer is able to be set to an objectedthickness by, for example, changing a remaining amount of the rollingoil at the cleaning process normally performed after the cold rolling,and changing the remaining nitrogen and the oxygen amount of the brightannealing furnace.

EXAMPLE

To verify effects of the present invention, the following tests areperformed.

At first, cold-rolled sheets of 25 μm to 150 μm in thickness aremanufactured as for one kind of pure titanium (thickness of 0.5 mm)defined by JISH4600 by passing through the cold rolling and anintermediate annealing. Subsequently, the finish annealing is performedwhile changing conditions in an Ar atmosphere (dew point≦−40° C.) tothereby change crystal grain sizes variously. Besides, the hardenedlayer is formed at a surface of the sheet by concentrating any ofoxygen, nitrogen, carbon by the rolling oil remained at the surface ofthe sheet and the gas atmosphere of the annealing furnace. The thickness(depth) of the hardened layer is adjusted by changing the remainingamount of the rolling oil and the nitrogen amount and the oxygen amountin the atmosphere at the bright annealing time.

Each of these cold-rolled sheets (each test piece) after the finishannealing is processed into a test piece with a parallel part of 6.25 mmin width, a parallel part of 50 mm in length, and thereafter, a tensiletest is performed. Besides, a sheet thickness, a crystal grain size,surface hardness, and a thickness of the hardened layer are measured asfor each test piece. Fe concentration (bulk mass %), O concentration(bulk mass %) of each test piece used for the example, and eachmeasurement result are illustrated together in Table 1.

TABLE 1 SHEET HARDENED SHEET CRYSTAL THICK- LAYER SURFACE 0.2% THICK-GRAIN NESS/ THICK- HARD- PROOF TENSILE ELONGA- Fe O NESS SIZE GRAIN NESSNESS STRESS STRENGTH TION (mass %) (mass %) (μm) (μm) SIZE (nm)(HV_(0.025)) (MPa) (MPa) (%) COMPARATIVE 0.04 0.05 25 NON- — 202 329 4205.9 EXAMPLE 1 RECRYSTAL- LIZED GRAIN EXAMPLE 1 0.04 0.05 25 2.6 9.6 320152 284 406 12.7 EXAMPLE 2 0.04 0.05 25 3.1 8.1 480 151 285 387 13.6EXAMPLE 3 0.04 0.05 25 3.7 6.8 450 156 248 365 15.5 EXAMPLE 4 0.04 0.0525 7 3.6 400 157 191 318 13.2 EXAMPLE 5 0.04 0.05 25 8.2 3 300 155 206336 15 COMPARATIVE 0.04 0.05 25 12.3 2 460 156 142 261 11.4 EXAMPLE 2COMPARATIVE 0.04 0.05 25 20.1 1.2 320 149 137 258 7.3 EXAMPLE 3COMPARATIVE 0.04 0.05 50 NON- — 190 362 459 13.9 EXAMPLE 4 RECRYSTAL-LIZED GRAIN EXAMPLE 6 0.04 0.05 50 2.8 17.9 470 145 281 432 22.4 EXAMPLE7 0.04 0.05 50 3.4 14.7 490 150 291 406 24.6 EXAMPLE 8 0.04 0.05 50 5.39.4 510 154 238 362 25 EXAMPLE 9 0.04 0.05 50 9.3 5.4 500 158 191 32924.1 EXAMPLE 10 0.04 0.05 50 12.4 4 500 152 168 299 22.6 COMPARATIVE0.04 0.05 50 19.9 2.5 510 147 151 271 18.4 EXAMPLE 5 COMPARATIVE 0.040.05 50 23.3 2.2 520 153 157 264 14.9 EXAMPLE 6 COMPARATIVE 0.03 0.03100 2.1 47.6 480 151 272 372 25.1 EXAMPLE 7 EXAMPLE 11 0.03 0.03 100 333.3 500 156 250 353 30.4 EXAMPLE 12 0.03 0.03 100 3.2 31.3 530 155 223354 30.2 EXAMPLE 13 0.03 0.03 100 4.7 21.2 450 151 187 341 33.7 EXAMPLE14 0.03 0.03 100 8.2 12.2 630 158 151 315 33 EXAMPLE 15 0.03 0.03 10011.6 8.6 1080 198 147 302 34.4 COMPARATIVE 0.03 0.03 100 15.6 6.4 2510262 216 306 26.3 EXAMPLE 8 EXAMPLE 16 0.03 0.03 100 17.9 5.6 1160 184145 303 35.2 EXAMPLE 17 0.03 0.03 100 31.3 3.2 1760 199 151 302 33COMPARATIVE 0.03 0.03 100 30.2 3.3 180 135 90 288 36.1 EXAMPLE 9COMPARATIVE 0.03 0.03 100 33.1 3 2300 242 182 298 27.7 EXAMPLE 10COMPARATIVE 0.03 0.03 100 38.7 2.6 1820 227 147 282 29.3 EXAMPLE 11COMPARATIVE 0.03 0.03 100 56.1 1.8 2430 254 167 278 25.2 EXAMPLE 12COMPARATIVE 0.03 0.03 150 1.9 78.9 410 171 279 388 30.1 EXAMPLE 13EXAMPLE 18 0.03 0.03 150 2.6 57.7 500 162 250 372 33.4 EXAMPLE 19 0.030.03 150 5.3 28.3 480 159 186 330 36.2 EXAMPLE 20 0.03 0.03 150 8.1 18.4420 160 158 311 35 EXAMPLE 21 0.03 0.03 150 12.5 12 410 155 148 302 40.6EXAMPLE 22 0.03 0.03 150 28.8 5.2 1810 246 150 289 39.5 COMPARATIVE 0.030.03 150 31.5 4.8 190 138 93 279 41.2 EXAMPLE 14 EXAMPLE 23 0.03 0.03150 45 3.3 1930 254 141 281 37.6 COMPARATIVE 0.03 0.03 150 44.6 3.4 160136 85 274 41.5 EXAMPLE 15 COMPARATIVE 0.03 0.03 150 52 2.9 2120 253 145257 28.7 EXAMPLE 16 COMPARATIVE 0.03 0.03 150 68.3 2.2 600 168 91 25228.3 EXAMPLE 17

The tensile test is performed in a direction (L direction) in parallelto a rolling direction, under conditions of a strain rate of 0.5%/min upto 0.2% proof stress, and thereafter, 20%/min up to fracture, under aroom temperature.

The crystal grain size is found by using the quadrature and squareapproximation as for a region of 40,000 μm² or more of a sample surface.

As for the surface hardness, a Vickers hardness meter is used, a Vickersindenter is pressed onto the sample surface with a load of 0.245 N (25gf), and it is evaluated by an average value of 10 points.

The thickness of the hardened layer is set to be a thickness in which adepth direction analysis of each of oxygen, nitrogen, carbon, titanium,and iron is performed by an Ar ion sputtering by using the GDS, at aregion of 4 mm in diameter of the sample surface, and any ofconcentrations of oxygen, nitrogen, and carbon, or a total concentrationof these becomes 0.5 mass % or more. As for quantification, eachmeasurement value is calibrated by using each of zinc oxide (oxygen:19.8 mass %) as for oxygen, austenitic stainless steel (nitrogencontent: 0.3 mass %) as for nitrogen, titanium alloy (carbon content:0.12 mass %) as for carbon, to be corresponded to a measurement portion(depth) of pure titanium (JIS one kind) to thereby perform the depthdirection analysis of each element.

In Table 1, characteristic values of the titanium material changedepending on the sheet thickness, contents (bulk concentration) of Fe,O, and therefore, they are each compared under approximately the samecondition. Besides, even when the sheet thickness, the contents of Fe, Oare the same, they are affected by the grain size, and therefore, thecomparison is performed in consideration of the grain size. Note that itis a problem in the shape retentivity in which the shape deforms bydeformation of a part where a processing amount is small, and therefore,the shape retentivity is able to be evaluated at a value of 0.2% proofstress at each sheet thickness.

A comparative example 1 and a comparative example 4 are both cases whennon-recrystallized grains remain, and the elongations are remarkablylow.

Each of comparative examples 2, 3, 5, 6, 11, 12, 16, 17 is a case when(the sheet thickness/the grain size) <3, and the elongation isremarkably low. In particular, the elongation of the comparative example17 is lower than examples 18 to 23.

A comparative example 7 and a comparative example 13 are both cases whenthe crystal grain sizes are too fine (less than 2.5 μm), and theelongations are low.

Each of comparative examples 8, 10, 12 is a case when the thickness ofthe hardened layer is larger than the thickness (200 nm or more and 2 μmor less) defined in the present invention, and the elongation is low. Inparticular, in the comparative example 12, the sheet thickness/the grainsize is less than 3, and the hardened layer is thick, and therefore, theelongation is lower than examples 11 to 17. In a comparative example 16,the sheet thickness/the grain size is less than 3, and the hardenedlayer is also thick, and therefore, the elongation is lower thanexamples 18 to 23.

In each of comparative examples 9, 14, 15, the thickness of the hardenedlayer is thin (less than 200 nm), 0.2% proof stress is low, and theshape retentivity is not good. In particular, in the comparative example14, the proof stress is remarkably low compared to the example 22 havingapproximately the same grain size. In the comparative example 15, theproof stress is remarkably low compared to the example 23 havingapproximately the same grain size.

When they are summarized by the same sheet thickness, results are asfollows.

“As for 25 μm material”

The comparative example 1 is a non-recrystallized structure, andtherefore, the elongation is low.

In each of the comparative examples 2, 3, the sheet thickness/the grainsize is less than 3, and the elongation, the proof stress, and thetensile strength are low compared to the examples 1 to 5.

“As for 50 μm material”

The comparative example 4 is the non-recrystallized structure, andtherefore, the elongation is low.

In each of the comparative examples 5, 6, the sheet thickness/the grainsize is less than 3, and the elongation, the proof stress, and thetensile strength are low compared to the examples 6 to 10.

“As for 100 μm material”

The comparative example 7 is too grain-refined, and therefore, theelongation is low.

In the comparative example 8, the sheet thickness/the grain size ≧3 issatisfied, but the hardened layer is thick, and the elongation is low.

In the comparative example 9, the hardened layer is thin, and the proofstress is remarkably low compared to the example 17 having approximatelythe same grain size.

In the comparative example 10, the hardened layer is thick, and theelongation is low compared to the examples 11 to 17.

In the comparative example 11, the sheet thickness/the grain size isless than 3, and the elongation is low compared to the examples 11 to17.

In the comparative example 12, the sheet thickness/the grain size isless than 3, the hardened layer is also thick, and therefore, theelongation is low compared to the examples 11 to 17.

“As for 150 μm material”

The comparative example 13 is too grain-refined, and therefore, theelongation is low.

In the comparative example 14, the hardened layer is thin, and the proofstress is remarkably low compared to the example 22 having approximatelythe same grain size.

In the comparative example 15, the hardened layer is thin, and the proofstress is remarkably low compared to the example 23 having approximatelythe same grain size.

In the comparative example 16, the sheet thickness/the grain size isless than 3, the hardened layer is also thick, and therefore, theelongation is low compared to the examples 18 to 23.

In the comparative example 17, the sheet thickness/the grain size isless than 3, and the elongation is low compared to the examples 18 to23.

On the other hand, each of the examples 1 to 23 is a case when theconditions defined in the present invention are satisfied, and exhibitshigh elongation and surface hardness.

INDUSTRIAL APPLICABILITY

The titanium thin sheet of the present invention includes excellentworkability and high surface hardness, and is able to be used for widepurposes as materials of consumer products and for industries such as,for example, a speaker vibration plate.

1. A titanium thin sheet of 0.2 mm or less in thickness, containing: Feof 0.1 mass % or less and O (oxygen) of 0.1 mass % or less in a bulk,wherein a sheet thickness (mm)/a grain size (mm) ≧3, and the grain size≧2.5 μm are satisfied, and a hardened layer is included at a surface,and a region of the hardened layer is a depth of 200 nm or more and 2 μmor less from the surface.
 2. The titanium shin sheet according to claim1, wherein after a cold-rolling, a finish annealing is performed at 500°C. or more and 850° C. or less by a BAF or a continuous annealing.